This research will examine how molecular (small-scale) variables affect choice behavior. One line of research will examine the effects of delay, such as when an individual first choose between a small but immediate work requirement and a larger but more delayed work requirement. A second line of research will study choice behavior in situations where the alternatives include one certain reinforcer and one uncertain reinforcer. A third line of research will examine choice behavior during Periods of transition--periods in which the reinforcement contingencies have just changed, and the subject crust adapt to the new circumstances. The subjects in this research will be pigeons performing in standard operant conditioning chambers with food as the reinforcer. One main procedure to be used in this research is an adjusting--delay procedure, in which the delay for one alternative is systematically increased and decreased over trials to find an indifference point-a delay at which the two alternatives are equally preferred. The studies on choice behavior in transition will use a procedure that collects many acquisition curves from each subject. The results from these studies will be used to test the predictions of different mathematical models, such as the "hyperbolic--decay model" of delayed reinforcement and several different theories of choice behavior in transition. They important everyday choices involve a conflict between an individual's short-term and long- term interests (e.g., the pleasures of smoking, drinking, or overeating versus future health). In addition, many everyday decisions bust be made when the consequences of one's actions are far from certain. The research described in this proposal is directed at developing a quantitative model for understanding how such choices are made, why individuals sometimes do and sometimes do not make choices that are in their best long-term interests, and what factors control these decisions.